Yeah, I actually covered this in my 2008 Tusken fanfic, "Crying Behind the Goggles". Kenobi was actually instrumental in spreading the slur, along with all kinds of lies. He would tell people the Tuskens, a proud warrior race, ate dried bantha dung, or they couldn't hold their space liquor. He was like the Custer to the Tuskens' native Americans. Not cool, man. I even included a part where he gives some Tuskens a bunch of blankets infected with Endorian lice.